Drag and drop capabilities for models and IRs

[AxillV]

As suggested in #445.

Currently the NAMDragDropControl class does not allow passthrough of clicks, therefore only one of: allowing drag and drop or using the plugin as normal is possible. I could add the onDrop on the individual NAMFileBrowserControl instances, but it wouldn't be as easy to use, in my opinion.

What's missing:

• [ ] Find a way to allow NAMDragDropControl to passthrough mousevents to controls below, or place it below and allow others to pass through onDrop.
• [ ] Add checks for valid file types, add support for IRs (couple lines of code).
• [ ] Check if file browser works as intended after dropping in a model.
• [ ] Add overlay to indicate to user that the plugin supports drag and drop, when holding a file over the plugin.

The first point is the one I'm mostly stuck in, everything else should be easy to implement. Any ideas?

[AxillV]

UPDATE: Currently not possible to implement, see here. Would we like to implement it only on the browsers, or wait for an implementation from IPlug's side? Don't know if I should mention a contributor so I'll leave this here for a bit.